In the Wilds of South America

by Leo E. Miller

It has been said that the females of this species lay eggs during a period of three or four months; to know how many are laid by a single bird would be interesting, as the number must be very great in order to make allowance for the incalculable numbers that are wasted, and still provide enough to keep the ranks of the multitudes at their normal level.

We did not find a single egg of M. b. bonariensis on the ground, although Hudson states that in the vicinity of Buenos Aires these birds “frequently waste their eggs by dropping them on the ground.”

Dropping the eggs on the ground might entail a deliberate waste, as we know of no reason why the bird should suppose that they would be hatched and the young reared, if scattered broadcast over the country. On the other hand, this might merely indicate that the birds had found no suitable place in which to deposit their eggs. The form of waste caused by the birds laying in old, disused nests, or by laying such a large number of eggs in a single nest that it is impossible for the rightful owner to incubate them and rear the young, can hardly be said to be deliberate, as it is doubtless caused by a lack of intelligence; if the bird designedly scatters its eggs broadcast on the ground, it is wantonly wasteful; if it merely lays in disused nests, or overcrowds nests actually occupied, the bird may simply be stupid.

It would be impossible to say what per cent of eggs laid by this species of cowbird is wasted. Hudson estimates that each female lays from sixty to one hundred eggs in a single season, and it does not seem to me that this statement is an exaggeration. One female which I dissected had laid three eggs within the few preceding days, and a fourth was almost ready to be deposited.

The bird which suffers most from the parasitic habits of the cowbird in the vicinity of Rosario de Lerma, is the oven-bird (Furnarius rufus); however, of the great number of eggs laid in the nests of the above-named species, our observations tend to show that the greater part are lost. Among the scores of oven-bird nests which we examined, only two were still occupied by the owners, the desertion being apparently due to the invasion of the cowbirds. So persecuted were the oven-birds that it is difficult to understand how any of them survived in this immediate locality. The nests were common enough, it being not unusual to find several of them in a single tree, but the birds themselves were not abundant. It is possible that some of the pairs may have built several nests each in their vain attempts to escape the attentions of the cowbirds.

In no instance had the walls or top of the oven-birds’ nests been broken or perforated in any manner, in order that light could penetrate to the interior; they were not tampered with in any way, and the cowbirds seemed content to use them just as the oven-birds had constructed them.

I believe that the greater number of M. b. bonariensis that reach maturity are reared by the smaller birds, such as finches, warblers, and vireos, in whose nests only a few eggs are laid, which increases the favorable chances of their incubation. Also, the larger and heavier eggs of the cowbird frequently crush at least a part of the smaller eggs which naturally have a more fragile shell, thus forestalling to a marked degree the competition that might arise between the young birds in the nest.

We collected about two hundred eggs of this species, nearly all of them at Rosario de Lerma, and a great variation in marking exists; there is also some difference in color. As a general rule the eggs are greenish or bluish, rather heavily spotted with reddish-brown; in a very few specimens the background is of a pale flesh-color, and in a small number of others it approaches white, having, however, a dull grayish tinge; of the entire lot, four only are so lightly marked as to appear unspotted. Not a single egg is pure white or has a pure white background (my standard of comparison is an egg of the oven-bird) “like the eggs of birds that breed in dark holes”; the majority of these eggs were taken from the darkened interiors of oven-birds’ nests.

A type of egg not uncommon is heavily and evenly marked all over with fine dots and larger spots of reddish-brown. Judging from the material at hand I should say that there is a characteristic type of marking running through the eggs of the species if we except the two extremes, viz., those almost unspotted, and those so entirely covered with heavy blotches that they appear to be of a uniform chocolate color.

However, the eggs of each individual seem to vary in some respect from those of any other, as it is impossible to find two exactly alike in comparing series from different places. Frequently, two or more eggs found in the same nest resemble each other so closely in size, shape, and coloration, that I think it reasonably safe to say that they were laid by the same bird.